The basic operation of digital voice links within a telephone system relies on parts of the telephone network operating with a common clock reference signal (typically based around 8 kHz). For example the local telephone exchanges at each end of a conventional telephone call will both use the same clock reference signal to ensure that voice signals converted from analogue to digital form at one exchange are converted back from digital to analogue form at the same rate. Therefore there is no over-run or under-run of digitized voice signals within the digital part of the phone network.
With the advent of digital services such as ISDN it is practical and desirable for the digital phone system to be extended to the user, rather than converting voice signals from analogue to digital form at the first telephone exchange.
By connecting personal computers to the digital telephone network users can make use of the network directly for voice, data and video conferencing. However for correct operation the timing reference signals must still be propagated within the PC to any devices generating or processing data for the digital telephone network. This invention provides inter alia a novel way of maintaining such timing within a PC.
Typically the communications within a personal computer for video conferencing is broken into two modules or logic blocks plugged into the ISA bus (or other standard PC bus). An ISDN network interface on a first card (defining a first logic block) recovers the serial bit stream from the public network. This bit stream then crosses a synchronous interface provided by a ribbon cable (i.e. not via the personal computer asynchronous bus) to a second card (defining a second logic block) that processes the received bit stream for display and compresses the camera input to be passed back along the synchronous interface to the ISDN link.
The synchronous link undertakes two basic functions:
1. Passing the transmit and receive data between the two cards (in this case about 16 Kbytes/sec in each direction). PA1 2. Propagating the timing reference from the public phone network, via the ISDN interface to synchronize the operation of the video codec.
The main problem with this known technique is the requirement for an additional cable to provide a synchronous link between the two cards. This leads to additional mechanical complexity and additional cost.